Electrical heating pad

ABSTRACT

An electrical heating pad, in particular a mattress to be placed under a patient during surgery, comprising a number of partially overlapping resistor elements (11-18). The foil elements cover different regions of the pad and are positioned in an overlapping configuration such that in substantially all portions of the pad there are at least two overlapping foil element parts. Each foil element is fed separately from control circuitry so as to maintain a uniform temperature at all portions of the pad, irrespective of the heating load at different portions.

FIELD OF THE INVENTION

The present invention relates to an electrical heating pad, inparticular a mattress, comprising a number of partially overlappingresistor foil elements.

BACKGROUND OF THE INVENTION

An electrically heated mat with partially overlapping resistor foilelements is known from U.S. Pat. No. 2,745,942. In this known mat, theresistor foil elements are disposed in three layers, one on top of theother, and the outline or outer contour of each element basicallycoincides with the contour of the mat. However, the individual resistorfoil paths are displaced or staggered relative to the foil paths of theother foil elements, so that the various foil paths supplement eachother and together cover substantially the whole surface area of themat. In this way, a uniform and intensive heating of the mat can beachieved.

SUMMARY OF THE INVENTION

The main object of the present invention, on the other hand, is toprovide an electrical heating pad which can be monitored in use so thatlocal heat variations, in particular hot spots, can be detected. Afurther object is to control the heating in such a manner that localheat variations, e.g., those caused by a non-uniform load, areeliminated. Thus, it is desirable to achieve a uniform temperaturedistribution over the pad even when the load or heat transfer isconcentrated to certain portions of the pad.

A special application, where such monitoring and control of the heat isimportant or even critical, is the use of the pad as a mattress for apatient during surgery. Of course, the normal physiological temperaturecontrol of the human body is then partially non-effective, and there isconsequently a need for a mattress which can monitor and preferablycontrol the temperature at all portions thereof.

According to the invention, the above-mentioned main object is met by anelectrical heating pad, in which the various foil elements coverdifferent regions of the pad and are positioned in such an overlappingconfiguration that in substantially all portions of the pad there are atleast two overlapping foil element parts, wherein each foil element insuch a pair of partially overlapping foil elements has a separate pairof feed terminals for electrically feeding the foil elements separatelyso as to permit individual sensing of the respectivetemperature-dependent resistance thereof. Thus, the foil elements have adual function, i.e., they serve as heating elements as well astemperature measuring elements, and no separate temperature sensingmeans, such as thermistors, are needed. If a hot spot occurs, thetemperature increase will be sensed by at least two foil elements, or atleast one if the other does not operate properly. Upon detection, thisinformation can be used to provide an alarm signal or to control theelectrical power supplied to the particular foil element or elements soas to maintain a uniform temperature distribution.

The invention also concerns electrical circuitry for electricallyfeeding and monitoring such a heating pad, the circuitry including anumber of circuit units connectable between a power source and each oneof the terminal pairs of the pad, each such circuit unit being adaptedto sense the temperature-dependent resistance of the corresponding foilelement, whereby the temperatures of the various foil elements aremonitored individually. Preferably, these circuit units are also adaptedto control the feeding voltage to the terminal pairs so as to keep allparts of the heating pad at a predetermined temperature value.

The partially overlapping arrangement of the foil elements in the pad,especially in combination with the electrical circuitry for feeding,monitoring and control thereof, provides excellent security. Thus, if aparticular foil element or the control thereof fails, this will be takencare of by the other, partially overlapping foil elements. In case theremaining operative foil elements cannot provide adequate compensation,the respective foil elements are preferably inactivated by means of asecurity switch. However, normally, reasonable variations in heattransfer from any part of the pad can be fully compensated for, at leastif the number of foil elements is relatively high so that the total areaof the pad is divided into a large number of local areas defined by aparticular pair of partially overlapping foil elements.

In this context, the "area" of a foil element is defined as the areaconfined within the outer contour of the resistor foil path, which mayhave any desired internal configuration, e.g., with meander-like,sinusoidal or labyrinthine loops distributed over the internal area in asubstantially uniform manner, as is known in the art. The term "foilelement" is defined herein primarily as a metal resistor foil stripembedded between two sheets of electrically insulating material,preferably a thermoplastic, flexible material. Alternatively, a "foilelement" can be made of a semiconductive, flexible material, such as PVCdoped with electrically conductive micro particles. Furthermore, theterms "overlapping" and "partially overlapping" concern the foilelements as a whole.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more clearly understood, a preferredembodiment will be described with reference to the appended drawings, inwhich

FIG. 1 shows a perspective view of a mattress according to the inventionwith partially cut-away parts;

FIG. 2 is a schematic block diagram illustrating the principal parts ofelectrical circuitry for monitoring and controlling the electrical powerbeing fed to the foil elements of the mattress of FIG. 1;

FIG. 3 is a schematic block diagram of the control and monitoring unitswithin the dotted lines in FIG. 2; and

FIG. 4 is a simplified circuit diagram illustrating one of the controlunits included in FIGS. 2 and 3.

DESCRIPTION OF PREFERRED EMBODIMENT

As illustrated in FIG. 1, a preferred embodiment of the electricalheating pad according to the invention is constituted by a generallyrectangular mattress to be placed under a patient lying on an operatingtable. In the illustrated example, the mattress has the dimensions900×450 mm so as to fit onto the central section of an operating table.

The mattress consists of two outer protective sheets 1,2 of a flexiblethermoplastic material, such as PVC, sealed hermetically all around thecircumference and enclosing therebetween, apart from possible layers ofa soft material (not shown), a number (e.g., eight) of partiallyoverlapping resistor foil elements 11-18 serving to heat the mattressuniformly to a desired temperature.

The foil elements 11-18 may be of a kind which is commerciallyavailable, wherein each foil element consists of a pair of rectangularthermoplastic flexible sheets embedding therebetween a metal foil stripfollowing a path of meander-like loops. Preferably, the foil strips aremade of brass having a temperature coefficient of about 0.15% per degreeC.

According to the invention, the foil elements 11-18 are dimensioned andarranged so as to cover different regions of the mattress and topartially overlap each other, so that in substantially all portions ofthe mattress there are two overlapping foil element parts. Inparticular, four foil elements 11, 12, 13 and 14 are positioned in thelongitudinal direction of the mattress, the two elements 11 and 12 beingplaced side by side in one longitudinal half of the mattress and the twoelements 13 and 14 being placed side by side in the other longitudinalhalf of the mattress. The remaining four foil elements 15,16,17 and 18are positioned in a transversal direction (perpendicularly to thelongitudinal direction), so that each of the transversal elements 15 and16 crosses and overlaps the two longitudinal elements 11 and 12, and thetransversal elements 17 and 18 cross and overlap the other twolongitudinal elements 13 and 14. Thus, the mattress is divided intoeight substantially square regions each defined by a particularcombination of foil element parts, namely 11/15, 11/16, 12/15, 12/16,13/17, 13/18, 14/17 and 14/18, respectively.

Furthermore, according to the invention, each foil element 11-18 has aseparate pair of electrical terminals 11a-b, 12a-b, 13a-b, 14a-b, 15a-b,16a-b, 17a-b and 18a-b brought together (although electrically insulted)into a common cable 3. Thus, the cable 3 includes 16 individual wires.In this way, it is possible to feed each foil element individually withan electrical current from a power source. Also, each foil element canbe monitored and controlled individually so as to maintain a uniformtemperature over the mattress, even if the heat transfer from themattress is unevenly distributed.

FIG. 2 illustrates a preferred embodiment of electrical circuitryadapted to provide such a monitored and controlled electrical feeding ofthe various foil elements 11-18 of the mattress. The circuitry comprisesa power supply unit 21 adapted to be connected to a regular AC currentsource (220 V) and to deliver a stabilized, lower DC current (24 V) viaa security switch 22 to eight control units C01-C08, each control unitfeeding a corresponding one of the foil elements 11-18. The controlunits C01-C08 are conncted to a monitor unit 23 adapted to sequentiallysense the resistance of the resistor strip in each foil element in orderto monitor the temperature of each foil element, the temperature beingdependent on the resistance. As indicated in FIG. 2, the power supplyunit 21, the control units C01-C08 and the monitor unit 23 are allconnected to an alarm unit 24 adapted to indicate any malfunctioning ofthe respective units by an audio or a visual signal. Also, the alarmunit 24 is adapted to activate the security switch 22 in case of suchmalfunctioning, so that the electrical power to all foil elements 11-18is switched off. Furthermore, a display unit 25 will indicate theaverage temperature of the foil elements 11-18.

The alarm unit will be activated basically in three different cases.Thus, if the power supply unit 21 does not operate properly, a signalP_(A) is delivered to the alarm unit 24. Furthermore, if any one of thecontrol units C01-C08 delivers too high a voltage to the correspondingfoil elements 11-18, an alarm signal U_(A) is delivered to the alarmunit. Finally, if the monitor unit 23 senses a resistance valuecorresponding to a temperature which is too high or too low in any oneof the foil elements 11-18, a temperature alarm signal T_(A) isdelivered to the alarm unit. In all these three cases, the power will beswitched off automatically by the security switch 22.

The control and monitor units are illustrated schematically in FIG. 3.The DC voltage obtained from the power supply via the security switch isapplied to each one of the parallelly connected control units C01, C02,. . . , C08, respectively. Each control unit is connected to thecorresponding terminal pair 11a-b, 12a-b, . . . , 18a-b, forelectrically feeding the respective foil element 11-18, and is adaptedto deliver three output voltages. A first output voltage U_(A) (feedingvoltage, Cf. FIG. 4) is applied to the alarm unit. In case this voltageexceeds a predetermined value, the alarm unit is activated primarily inorder to protect the various electronic components in the control units.The second output voltage U_(R1), U_(R2), . . . , U_(R8), respectively,is applied to a first, sequentially operating switch S1, whereas a thirdoutput voltage U_(r1), U_(r2), . . . , U₈, respectively, is applied to asecond, sequentially operating switch S2. The two switches S1 and S2,which belong to the monitor unit 23 (FIG. 2), are synchronized so as toapply corresponding pairs of these output voltages to a voltage divider30, the output of which, U_(Rn) /U_(rn), n=1,2, . . . , 8, isproportional to the temperature T_(n) of the corresponding foil element11-18. The temperature signal T_(n) is applied to two integrators RC1and RC2 connected in parallel to each other. The first integrator RC1has a relatively long time constant, and the output T_(m) thereofindicates the average temperature value of the foil elements 11-18 andis applied to the display unit 25 for indicating the average temperatureof the mattress. The second integrator RC2, on the other hand, has arelatively short time constant, and the output T_(A) thereof correspondsto the actual temperature of the particular foil element and is appliedto the alarm unit. If this indicated temperature value at any momentexceeds a first predetermined value, such as 42° C., or is lower than asecond predetermined value, such as 15° C. (probably due to electronicmalfunctioning), the alarm unit will activate the security switch fordisconnecting the electrical feeding of the foil elements.

The principal structure of each control unit C01-C08 is illustrated inFIG. 4. A voltage controller 40, which includes a switching element anda LC-filter providing a DC-DC-conversion without substantial heat loss,controls the feeding voltage by means of an operational amplifier OPinserted in a feed-back loop. The output voltage of the voltagecontroller 40 is applied to a bridge containing the corresponding foilresistor strip R_(n), n=1,2, . . . , 8, between the terminals a and b,and a reference resistor r_(n), n=1, 2, . . . , 8. The terminal bbetween the foil resistor R_(n) and the reference resistor r_(n) isconnected to the negative input of the operational amplifier OP, whereasthe positive input of the operational amplifier OP is connected to areference voltage U_(ref), which is manually adjustable for setting apredetermined temperature value (common to all foil elements).

During operation, the resistance of the foil resistor R_(n) will varylinearly with the temperature thereof, but the operational amplifier OPand the voltage controller 40 controls the feeding voltage in such a waythat the operating temperature of the particular foil element will bemaintained substantially at the preset temperature value correspondingto U_(ref). As can be easily calculated, the resistance of the foilresistor R_(n) is equal to r_(n) (U_(Rn) /U_(rn) -1). Thus, the voltageobtained at the output of the divider 30 (FIG. 3) corresponds to theresistance of the foil resistor R_(n) and hence the temperature T_(n)thereof.

The mattress and the control circuitry described above can be modifiedin many ways by those skilled in the art. Thus, the dimensions and themechanical structure of the mattress can be adjusted in view of therequirements set forth by a particular application. Furthermore, thegeometrical dimensions and the overlapping arrangement of the foilelements can also be modified.

In case a larger number of foil elements is used, the control andmonitor units of the electrical circuitry may be partially replaced by amicroprocessor or a computer.

Moreover, the "pad" does not have to be rectangular or planar but mayhave any suitable configuration, e.g., tubular (to enclose an arm or aleg) or even formed as a garment.

Finally, a plurality of applications outside the medical field arefeasible, e.g., the use of the pad for room heating, preferably bydisposing the pad in the floor of a room. In such a case, the pad willautomatically provide an appropriate heat transfer with reduced heatingpower at those portions being covered by heat insulating objects, suchas mats or pieces of furniture.

I claim:
 1. An electrical heating pad, comprising a number of partiallyoverlapping resistor foil elements (11-18), each such foil elementhaving a separate pair of electrical feed terminals (11a-b, . . . ,18a-b), wherein(a) substantially the whole area of the pad is dividedinto a plurality of sub-regions; (b) the foil elements (11-18) arepositioned in an overlapping configuration such that in each subregion(11/15, 11/16, 12/15, 12/16, 13/17, 13/18, 14/17, 14/18) of the padthere is a unique pair of overlapping foil element parts; and (c) thefeed terminals (11a-b, . . . , 18a-b, respectively) of each foil elementin such a pair of partially overlapping foil elements is connectable toan electrical power unit so as to permit individual sensing of therespective temperature-dependent resistance thereof, whereby thetemperature of each sub-region of the pad can be monitored individually.2. A heating pad as claimed in claim 1, wherein the foil elements(11-18) are elongated and oriented in different directions so as topartially overlap each other.
 3. A heating pad as claimed in claim 2,wherein the foil elements (11-18) are rectangular and oriented inmutually perpendicular directions.
 4. A heating pad as claimed in claim3, wherein the heating pad is divided into rectangular or squareportions, each such portion being divided into at least four of saidsub-regions, and including at least two foil elements (e.g., 15, 16)extending in a second direction perpendicular to said first direction.5. An electrical power unit for electrically feeding and monitoring aheating pad as claimed in any one of claims 2 to 4 and 1, wherein thepower unit includes a number of circuit units (C01-C08) beingconnectable between a power source (20) and each one of said foilelement feed terminal pairs (11a-b, . . . , 18a-b), each such circuitunit (C01-C08) being adapted to sense the temperature-dependentresistance (R_(n)) of the corresponding foil element, whereby thetemperature (T_(n)) at each of said sub-regions is monitoredindividually.
 6. An electrical power unit as claimed in claim 5, whereinsaid circuit units (C01-C08) are also adapted to control the electricalfeeding (U_(Rn) -U_(rn)) to said terminal pairs (11a-b, . . . , 18a-b)so as to keep all parts of the heating pad at a predeterminedtemperature.
 7. An electrical power unit as claimed in claim 6, whereineach control unit (C01-C08) comprises a comparator (OP), a first inputof which is connected to a voltage (U_(rn)) corresponding to the feedingvoltage and a second input of which is connected to an adjustablereference voltage (U_(ref)) corresponding to a desired temperature, andthe output of which is connected to a voltage controlling device (40).8. An electrical power unit as claimed in claim 7, wherein said voltagecontrolling device (40) includes a switching element coupled to a filterproviding a stable, variable controlling voltage which is applied to therespective foil element terminal pair.
 9. An electrical power unit asclaimed in claim 5, wherein said circuit units (C01-C08) are adapted todetermine the resistance (R_(n)) of each foil element by sensing thevoltages (U_(Rn),U_(rn)) in a bridge, which comprises the foil elementresistor (R_(n)) and a reference resistor (r_(n)) connected in series,and calculating the quotient (U_(Rn) /U_(rn)) of the voltage values(U_(Rn), R_(rn)) obtained at each side of the foil element resistor(R_(n)).
 10. An electrical power unit claimed in claim 9, wherein amonitor unit (23) is adapted to sense said circuit units (C01-C08)sequentially, said monitor unit including a divider (30) for dividingthe two voltage values (U_(Rn),U_(rn)) sensed by each circuit unit. 11.An electrical power unit as claimed in claim 5, thetemperature-depending output signal (T_(n)) is applied, on the one hand,via an integrator (RC1) having a relative long time constant to anindicator (25) for indicating the average temperature (T_(m)) of thefoil elements and, on the other hand, via an integrator (RC2) having arelatively short time constant to a detector (24) detecting criticallyhigh or low temperature values of a single foil element.
 12. Anelectrical power unit as claimed in claim 11, wherein said detector isconnected to an alarm unit (24) and/or a security switch (22) adapted tointerrupt the feeding power to at least one foil element (11-18) in casea critically high or low temperature has been detected.